Sub-volt drive 100 GHz bandwidth electro-optic modulator

ABSTRACT

Electro-optical modulators and methods of fabrication are disclosed. An electro-optical modulator includes a Mach-Zehnder interferometer formed in a substrate removed semiconductor layer and a coplanar waveguide. Signals from the coplanar waveguide are capacitively coupled to the Mach-Zehnder interferometer through first and second dielectric layers having strong dielectric constant dispersion.

RELATED APPLICATION INFORMATION

This application is a National Stage Application filed under 35 U.S.C.371 and claims the benefit of priority to Patent Cooperation TreatyApplication No.: PCT/US2014/042154 filed Jun. 12, 2014, which claimsbenefit of priority from U.S. Provisional Patent Application No.61/834,788, filed Jun. 13, 2013, titled SUB-VOLT DRIVE 100 GHZ BANDWIDTHELECTRO-OPTIC MODULATOR. The full contents of the International PatentApplication are incorporated herein by reference.

GOVERNMENT INTERESTS

This invention was made with Government support under Grant No.08ST1-0221 awarded by Defense Advanced Research Projects Agency-SmallBusiness Technology Transfer Program. The Government has certain rightsin the invention.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND

1. Field

This disclosure relates to optical modulators for application in fiberoptic communications systems and other electro-optic systems.

2. Description of the Related Art

Optical modulators are essential components for fiber opticcommunication, RF photonics, instrumentation and optical signalprocessing applications. Desired characteristics of optical modulatorsare a modulation bandwidth as high as possible while maintaining a drivevoltage as low as possible. Usually achieving these two propertiesrequires conflicting sets of design rules. For example ultra-low drivevoltage can be achieved using substrate removed (SURE) waveguides(JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli, “0.3 V drive GaAs/AlGaAssubstrate removed Mach-Zehnder intensity modulators,” Appl. Phys.Letters, vol. 92, 201103, 2008). SURE waveguides are formed in asemiconductor epitaxial layer that is removed from its originalsubstrate. Both sides of the epitaxial layer can be processed, enablingvery novel designs. Such submicron thick waveguides have very highvertical index contrast and can guide the optical wave with very lowloss of about a few dB/cm (JaeHyuk Shin, Yu-Chia Chang, and Nadir Dagli,“Propagation loss study of very compact GaAs/AlGaAs substrate removedwaveguides,” Optics Express, Vol. 171, No. 5, 2009). However metalelectrodes cannot be used on such waveguides since overlap of theoptical mode with the metal typically results in excessive opticalpropagation loss. This difficulty is circumvented using dopedsemiconducting layers as buried electrodes. Therefore very strongelectric fields overlapping very well with optical mode in the waveguidecan be generated. resulting in ultra-low drive voltage. However finitesheet resistance of the doped semiconductor layers may create excessiveresistance and electrode loss which in turn limits the bandwidth of themodulator to about 30 GHz (Selim Dogru, JaeHyuk Shin and Nadir Dagli,“Wide Bandwidth Design of Ultra-Low Voltage Substrate-RemovedElectro-Optic Mach-Zehnder Intensity Modulators,” Integrated Photonicsand Nanophotonics Research Conference Proceedings, Paper IWA6, Honolulu,Hi., Jul. 12-17, 2009; Selim Dogru, Jae Hyuk Shin and Nadir Dagli,“Traveling Wave Electrodes for Substrate Removed Electro-opticModulators with Buried Doped Semiconductor Electrodes” IEEE J. QuantumElectron., vol. 49, No. 7, pp. 599-606, July 2013).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an electro-optic modulator.

FIG. 2 is a cross-sectional schematic view of one optical waveguide ofthe electro-optic modulator.

FIG. 3A is a plan view of a portion of the electro-optic modulator.

FIG. 3B is a cross-sectional schematic view of the electro-opticmodulator.

FIGS. 4A, 4B, 4C, and 4D are cross-sectional views of one opticalwaveguide of the electro-optic modulator illustrating an exemplaryfabrication process.

FIG. 5 is an electrical equivalent circuit of a coplanar waveguide.

FIG. 6 is a cross-sectional schematic view of one optical waveguide ofthe electro-optic modulator showing equivalent circuit elements.

FIG. 7 is an electrical equivalent circuit of a portion of theelectro-optic modulator.

FIGS. 8A, 8B, and 8C are graphs showing the simulated performance of theelectro-optic modulator

Throughout this description, elements appearing in figures are assignedthree-digit reference designators, where the most significant digit isthe figure number and the two least significant digits are specific tothe element. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION Description of Apparatus

A novel design for an ultra-wide bandwidth very low drive voltagemodulator is presented. This design combines buried electrodes made ofdoped semiconductors and dielectrics with very large dielectric constantdispersion. This approach bypasses the bandwidth limit due to largeelectrode loss originating from buried electrodes at microwavefrequencies while allowing very low drive voltage due to much reducedelectrode gap resulting from buried electrodes. A device with 0.4 voltor less operating voltage and bandwidth exceeding 100 GHz is possible.

Referring now to FIG. 1, an ultra-wide bandwidth very-low drive voltagemodulator 100 may include a Mach-Zehnder interferometer 110 fabricatedon a substrate 105. A Mach-Zehnder interferometer is a well-knownwaveguide device comprised of an input optical waveguide 112, a splitter113, two branch optical waveguides 114, a combiner 115, and an outputoptical waveguide 116. The optical waveguides 112-116 are shown asdashed lines in FIG. 1. All optical waveguides 112-116 support a singlepropagating mode. Light coupled into the input optical waveguide 112 isdivided between the two branch optical waveguides 114 by the y-branchsplitter 113, which could also be a directional coupler or a multi-modeinterference coupler. After transiting the two branch optical waveguides114, the light is recombined and directed into the output opticalwaveguide 116 by the y-branch combiner 115 which could also be adirectional coupler or a multi-mode interference coupler. The light isdelayed or phase-shifted by the transit along the two branch opticalwaveguides 114. When the phase shifts in the two branch opticalwaveguides 114 are equal, a maximum amount of light is directed into theoutput optical waveguide 116 (i.e., 100% less any losses inherent in thewaveguides). When the phase shift between the two branch opticalwaveguides 114 is 180 degrees, the light from the two branch opticalwaveguides 114 interferes at the combiner 115, and little light isdirected into the output optical waveguide 116.

A Mach-Zehnder interferometer may be fabricated on an electro-opticalmaterial, which is to say a material where the index of refraction canbe varied by applying an electric field. When a Mach-Zehnderinterferometer, such as the Mach-Zehnder interferometer 110, is used asa modulator, a time-varying electric field may be applied to one or bothof the two waveguide branches to vary the index of refraction of thewaveguide branches. Varying the index of refraction changes the phaseshift incurred by light transiting the waveguide branches, and thusmodulates the amount of light coupled into the output waveguide.

In the ultra-wide bandwidth very-low drive voltage modulator 100,electric fields are applied to the two branch optical waveguides 114 bya coplanar transmission line 120 superimposed on the Mach-Zehnderinterferometer. The coplanar transmission line 120 includes a signaltrace 124 (S) between two ground traces 122, 126 (G). A microwave signalintroduced at one end of the coplanar transmission line 120 willpropagate along the waveguide to the other end. Commonly, the signal maythen be dissipated in a load (not shown) that matches the impedance ofthe coplanar waveguide. A coplanar strip can also be used in place ofthe coplanar transmission line.

The ultra-wide bandwidth very-low drive voltage modulator 100 as anexample of what is commonly called a traveling wave modulator, since thelight exiting the Mach-Zehnder interferometer is modulated by anelectric wave as the wave travels down the coplanar transmission line.In order to effect maximum modulation of the light, the propagationvelocity of the microwave signal in the coplanar transmission line 120needs to be the same as the propagation velocity of the light in thebranch optical waveguides 114.

The ultra-wide bandwidth very-low drive voltage modulator 100 may beformed in a substrate removed compound semiconductor layer. In thispatent, a “substrate removed semiconductor layer” is a semiconductorlayer that is attached to a supporting substrate by an adhesive, asopposed to a semiconductor layer that is grown on, or deposited directlyon, a substrate. The adhesive may be, for example, Benzocyclobutane(BCB) or another polymer material. The substrate removed compoundsemiconductor layer may be initially created as an epitaxial layer on asubstrate, and then removed from the substrate and glued onto a transfersubstrate using a glue such as BCB. Details of the transfer andsubstrate removal processes will be provided subsequently.

FIG. 2 shows a schematic cross-section view of one of the branch opticalwaveguides 114 of the Mach Zehnder interferometer 110. The opticalwaveguide 114 may include a central intrinsic (i) layer 210 composed ofInAlAs/InAlGaAs multi quantum wells (MQWs). The central i-layer 210 mayalso be any other material having an electro-optic effect, such as GaAs,InP, Si or other compound semiconductor MQWs. The central i-layer 210may be sandwiched between a p-doped layer 220 and an n-doped layer 225.The p-doped layer 220 and the n-doped layer 225 may be, for example, InPor other suitable p and n doped compound semiconductor or siliconlayers. The p-doped layer 220, the central i-layer 210, and the n-dopedlayer 225 form a p-i-n diode. The positions of the p and n doped layers220, 225 may be interchanged. In other words 220 could be the n dopedlayer and 225 could be the p doped layer.

The p-doped layer 220, the central i-layer 210, and the n-doped layer225 also form a so-called “staircase waveguide” 200 (indicated by thedashed oval) in a region where the p-doped layer 220 and the n-dopedlayer 225 overlap. A similar staircase waveguide may also be formedusing other electro-optic materials such as bulk GaAs, InP or Si. Astaircase waveguide is similar to a rib waveguide but each side of therib is etched on opposite sides of the substrate removed semiconductorlayer. The staircase waveguide may support a single mode waveguide at awavelength of operation of the ultra-wide bandwidth very-low drivevoltage modulator 100. The single optical mode will have a Gaussian-likeelectric field distribution and will not be precisely confined withinthe waveguide 200.

The refractive index of the central i-layer 210 (and thus the phaseshift of light propagating along the waveguide 200) may be varied byapplying an electric field across the a p-i-n diode formed by thep-doped layer 220, the central i-layer 210, and the n-doped layer 225.Electrical connections may be made to the p-doped layer 220 and then-doped layer 225 by a first ohmic contact 240, and a second ohmiccontact 245 on the sides of the waveguide away from the optical mode.However, when a microwave modulating signal is applied between the ohmiccontacts 240, 245, AC currents will still flow laterally through thep-doped layer 220 and the n-doped layer 225. The resistance of thep-doped layer 220 and the n-doped layer 225 and the capacitance of thecentral i-layer 210 will, in over simplified terms, form a low passfilter that may limit the bandwidth of ultra-wide bandwidth very-lowdrive voltage modulator 100 due to attenuation of the microwave signalfrom resistive losses in the p-doped layer 220 and the n-doped layer225.

To increase the bandwidth of the ultra-wide bandwidth very-low drivevoltage modulator 100, first and second metal conductors 250, 255 mayextend respectively from the first and second ohmic contacts 240, 245.The first and second metal conductors 250, 255 may overlap the p-dopedlayer 220 and the n-doped layer 225 respectively. However, to avoidattenuation of the optical mode traveling in the waveguide 200, theoptical mode should not overlap the first and second metal conductors250, 255 or the first and second ohmic contacts 240, 245. Thus the firstand second metal conductors 250, 255 may be separated from the p-dopedlayer 220 and the n-doped layer 225 by respective first and seconddielectric layers 230, 235. The thickness of the first and seconddielectric layers 230, 235 may be sufficient to ensure that the firstand second metal conductors 250, 255 do not overlap the optical mode.The first and second metal conductors 250, 255 may be capacitivelycoupled to the p-doped layer 220 and the n-doped layer 225 through therespective first and second dielectric layers 230, 235.

The first and second dielectric layers 230, 235 between first and secondmetal conductors 250, 255 and the p-doped and n-doped layers 220, 225may be made from a dielectric material having a high dielectric constantat microwave and millimeter wave frequencies but a low refractive indexat optical frequencies. A high dielectric constant at microwave andmillimeter wave frequencies may ensure strong AC coupling from the firstand second metal conductors 250, 255 to the p-doped and n-doped layers220, 225. The use of a dielectric material with a low dielectricconstant and correspondingly low index of refraction at opticalfrequencies may assist in confining the optical mode such that theoptical mode does not overlap the first and second metal conductors 250,255 or the first and second ohmic contacts 240, 245. To confine theoptical mode, the index of refraction of the first and second dielectriclayers 230, 235 may be lower than the index of refraction of the centrali-layer 210 and less than the index of refraction of the p-doped andn-doped layers 220, 225. (about 3.1 at 1.55 microns).

For example, a central i-layer 210 composed of InAlAs/InAlGaAs multiquantum wells will have an index of refraction about 3.5 at a wavelengthof 1.55 microns. InP p-doped and n-doped layers 220, 225 will have anindex of refraction about 3.1 at 1.55 microns.

The refractive index n of a material is given by the following equation:n=√{square root over (∈_(r)μ_(r))}where ∈_(r) is the relative permittivity or dielectric constant of thematerial and μ_(r) is the relative permeability of the material. Fornon-magnetic material, μ_(r) is very close to 1.0. Thus the relationshipbetween dielectric constant and refractive index of the dielectriclayers 230, 235 may be given byn=√{square root over (∈_(r))} or ∈_(r) =n ².The first and second dielectric layers 230, 235 may be a dielectricmaterial having strong dielectric constant dispersion, which is to saythe dielectric constant of the material at microwave and millimeterfrequencies may be substantially larger than the square of therefractive index of the material at optical frequencies. A dielectricmaterial is considered to have strong dielectric constant dispersion ifthe following equation is satisfied:∈_(rμ)>5n _(o) ²,where ∈_(rμ) is the dielectric constant of the material at a microwavefrequency of operation of ultra-wide bandwidth very-low drive voltagemodulator 100 and n_(o) is the index of refraction for an opticalwavelength of operation of the ultra-wide bandwidth very-low drivevoltage modulator 100.

Some examples of materials suitable for the first and second dielectriclayers 230, 235 are LiNbO₃, Ta₂O₅, and BaTiO₃. For example, bulk BaTiO₃,has microwave dielectric constant of 2200. In thin films, the microwavedielectric constant of BaTiO₃ reduces to about 200. BaTiO₃, has an indexof refraction of about 2.3 for an operating wavelength of 1.55 μm. Anindex of refraction of 2.3 is low enough to provide rapidly decayingoptical mode in the dielectric layers 230, 235. For an opticalwavelength of 1.55 μm, the dielectric layers 230, 235 may be 0.6 μmthick to confine the mode such that optical mode does not overlap withthe first and second metal conductors 250, 255. The index of refractionof LiNbO₃ is about 2.2 at an optical wavelength of 1.55 microns and themicrowave dielectric constant of thin film LiNbO₃ is about 35.

A top schematic view of a portion of the ultra-wide bandwidth very-lowdrive voltage modulator 100 is shown in FIG. 3A and a cross-sectionalschematic view of the same portion is shown in FIG. 3B. In FIGS. 3A and3B branches are connected in parallel between signal and ground traces.They can also be connected in series. This arrangement may require acoplanar strip rather than a coplanar transmission line.

As shown in FIGS. 3A and 3B, two optical waveguides 114L, 114R formed inthe substrate removed semiconductor layer are positioned adjacent to thegaps between the signal electrode 124 and ground electrodes 122, 126 ofa coplanar transmission line. The designators “L” and “R” refer to theleft-hand and right-hand waveguides as shown in this drawing and have noother physical meaning. Short stubs extending from the signal electrode124 and ground electrodes 122, 126 connect to modulator electrodes 305that overlap the optical waveguides 114. Note that modulator electrodes315 shown as dashed lines are located beneath the optical waveguides114L, 114R. Modulator electrodes extending from the ground electrode 122are disposed on top of the left-hand optical waveguide 114L. Modulatorelectrodes extending from the signal electrode 124 are disposed beneaththe left-hand optical waveguide 114L and on top of the right-handoptical waveguide 114R. Modulator electrodes extending from the groundelectrode 126 are disposed beneath the right-hand optical waveguide114R.

The waveguides optical waveguides, 114R are periodically implanted toform short semi-insulating sections 310. The implant material may beboron, protons, oxygen or any other suitable material. Thesesemi-insulating sections 310 provide electrical isolation betweenadjacent modulator electrodes 305, 315. These semi-insulating sections310 ensure that currents cannot flow through the p-doped and n-dopedlayers 220, 225 along the length of the modulator. Hence these shortmodulator electrodes 305, 315 and the interposed lengths of (notimplanted) p-i-n diode formed by the optical waveguides 114L, 114R formsmall capacitive elements that periodically load the coplanartransmission line, increasing its capacitance per unit length. Themodulator electrodes 305, 315 may be configured to adjust thecapacitance per unit length of the coplanar transmission line toequalize the group velocity of the coplanar transmission line and theoptical waveguides 114L, 114R.

FIGS. 4A, 4B, 4C, and 4D illustrate a simplified process for fabricatinga modulator, such as the ultra-wide bandwidth very low drive voltagemodulator 100. As shown in FIG. 4A, the p-doped layer 220, the centrali-layer 210, and the n-doped layer 225 may be sequentially grown on thesurface of a compound semiconductor substrate 400. Subsequently, thesemiconductors layers 220, 210, 225 may be patterned, and the secondohmic contract 245, the second dielectric layer 235, and the secondmetal conductor 255 may be formed. The p-doped layer 220 may beselectively implanted to form semi-insulating sections as previouslydescribed (not shown). The compound semiconductor substrate 400 may thenbe bonded face down to a semi-insulating GaAs or InP (or any otheroptically flat and insulating material) substrate 105 using adhesive205, as shown in FIG. 4B. The adhesive 205 may be, for example, BCB oranother polymer material. The original compound semiconductor substrate400 may then be removed using, for example, a selective wet or dry etchprocess, the result of which is shown in FIG. 4C. Finally, as shown inFIG. 4D, the upper layers of the optical waveguides and the coplanartransmission line may be formed. These layers include the firstdielectric layer 230, the first ohmic contact 240, and the firstconductor 250.

When a microwave signal propagates along the coplanar transmission line120 of the ultra-wide bandwidth very-low drive voltage modulator 100,there will be index changes in the substrate removed compoundsemiconductor layer due to linear electro-optic (LEO), quadraticelectro-optic (QEO) and free carrier (FC) effects. The resultantdifferential phase shift between the arms of the interferometer isproportional to the difference of the index changes in each arm. Duringthe operation, arms of the interferometer are biased such that the sameDC bias resulting in a bias field of E_(B) exists across each p-i-ndiode in each arm. But the polarity of the AC field is changed betweenthe arms due to physical electrode connection shown in FIG. 3B. Hencesame magnitude and opposite direction AC fields of E_(AC) exist in eacharm. As a result, biasing electric field across the arms areE_(B)+E_(AC) and E_(B)−E_(AC). Then

${{\Delta\; n_{L\; E\; O}} = {\frac{1}{2}\frac{n_{m}^{4}}{n_{e}}r_{41}2\; E_{AC}\Gamma_{L\; E\; O}}},{{\Delta\; n_{Q\; E\; O}} = {\frac{1}{2}\frac{n_{m}^{4}}{n_{e}}R\; 4\; E_{Bias}\Gamma_{Q\; E\; O}E_{AC}\mspace{14mu}{and}}}$Δ n_(F C) = K_(N)Δ N^(x)Γ_(N) + K_(P)Δ P^(y)Γ_(P) where n_(m) is the material index, n_(e) is the effective index of themode, r₄₁ is the LEO coefficient, R is the QEO coefficient, Γ_(LEO) andΓ_(QEO) are the overlap factors of the optical mode with the electricfields appropriate for the LEO and QEO effects, Γ_(N) and Γ_(P) are theoverlap factors of the optical mode with the depleted n and p layers andK_(N), K_(P), x and y are the appropriate parameters for a givenmaterial. The index change needed to create a π phase shift between thearms of the modulator is Δn_(NET)=λ/(2LF), where L is the length of thearms (see FIG. 1) and F is the fill factor. F arises since modulatorarms are segmented using ion implantation due to high speedconsiderations and only a certain fraction of the electrode is active.In this case F=p/d. Furthermore E_(B)=V_(Bias)/t and E_(AC)=V_(AC)/t.The reason why E_(AC) depends on the thickness t of the MQW and not onthe separation between the top and bottom metal electrodes is a keyfeature of the ultra-wide bandwidth very low drive voltage modulator andis explained in the next section. Substrate removal enables very small tvalues while keeping the overlap factors Γ_(LEO) and Γ_(QEO) high due tovery high vertical optical confinement. Furthermore optical propagationloss is also low hence a reasonably long device can be fabricated.Calculations indicate that a 3 mm long device with a 45% fill factor and0.3 μm thick MQW i-region has a V_(π) of 0.4 V. Recent experimentalresults gave V_(π) of 0.2 V for a 3 mm long device with 100% fill factorsupporting this calculation (Selim Dogru and Nadir Dagli, “0.2 V DriveVoltage Substrate Removed Electro-Optic Mach-Zehnder Modulators with MQWcores at 1.55 μm, “IEEE/OSA J. of Lightwave Technology, vol. LT-32, NO.3, pp. 435-439, Feb. 1, 2014).

The bandwidth of the ultra-wide bandwidth very low drive voltagemodulator 100 may be determined by the microwave and millimeter wavecharacteristics of the coplanar transmission line. In traveling waveoperation, widest bandwidth is obtained when optical and microwave groupvelocities are matched. The coplanar transmission line should not haveany dispersion or its group and phase velocities should be the same.Even under perfect velocity matching, 3-dB bandwidth is reached whenelectrode loss becomes 6.4 dB. An electrical equivalent circuit of thecoplanar transmission line is as shown in FIG. 5. Here L_(u), C_(u),R_(u) and G_(u) are the inductance, capacitance, resistance andconductance per unit length of the unloaded coplanar line. This coplanartransmission line is periodically loaded by electrically isolatedmodulator sections having admittance Y_(M) and the accurate modeling ofthese sections is essential.

A model of the modulator sections can be developed as shown in FIG. 6and the equivalent circuit shown in FIG. 7. Each modulator section isabout 100 μm long. So it can be modeled using lumped equivalent circuitelements. We take slices of Δx wide in the cross section and model eachslice using appropriate resistance and capacitance per unit width ofeach slice. R_(Metal) is the resistance per unit width of the first andsecond conductors 250, 255. R_(InP) is the resistance per unit width ofthe p-doped layer 220 and the n-doped layer 225. C_(Dielectric) is thecapacitance per unit width of the first and second dielectric layers230, 235. C_(MQW) is the capacitance per unit width of the centrali-layer 210. In this circuit R_(Metal)<<R_(InP) andC_(Dielectric)>>C_(MQW) due to very low sheet resistance of the metaland very high dielectric constant of the dielectric. At DC and lowfrequencies an external voltage applied to the ohmic contacts 240, 245is applied across the central i-layer 210 through the p-doped layer 220and the n-doped layer 225 that act as buried electrodes. Due to finitesheet resistance of the p-doped layer 220 and the n-doped layer 225,ohmic losses arise, increasing the microwave loss of the electrode. Thisloss will eventually limit the bandwidth. However as frequencyincreases, impedance of C_(Dielectric) is very low, effectivelyconnecting the ohmic contacts 240, 245 to the p-doped layer 220 and then-doped layer 225 through very low impedance R_(Metal) andC_(Dielectric). Hence the external voltage appears across the centrali-layer 210 uniformly and the current flow in the p-doped layer 220 andthe n-doped layer 225 is significantly limited. Therefore losses arisingfrom these doped buried electrodes are reduced significantly. This inturn enhances the bandwidth drastically.

Modulator electrode characteristics and modulation response werecalculated using the equivalent circuits shown in FIG. 5 and FIG. 7. Forunloaded line modeling, existing empirical formulas are used. In thesimulations, W=100 μm, g=25 μm and the BCB is 10 μm thick. FIGS. 8A, 8B,and 8C respectively show the characteristic impedance, Z_(c), andattenuation coefficient, α, of the electrode as well as the modulationresponse of the modulator up to 100 GHz for different relativedielectric constants, ∈_(r), of the dielectric. α and Z_(c) show milddependence on ∈_(r). A tenfold increase in ∈_(r) changes thesequantities by about 10% and most of the change takes place when∈_(r)≦200. Hence there is no need to obtain extremely large ∈_(r)values. Modulation bandwidth exceeds 100 GHz for ∈_(r) greater thanabout 150, demonstrating the ultra-wide bandwidth of this design withvery low drive voltage. FIG. 8C also shows the result of a simulationwithout the dielectric and metal layers. In this case bandwidth is about5 GHz. The techniques described herein improve the bandwidth more than15 times.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An electro-optical modulator comprising: aMach-Zehnder interferometer formed in a substrate removed semiconductorlayer; and a coplanar waveguide, wherein signals from the coplanarwaveguide are capacitively coupled to the Mach-Zehnder interferometerthrough first and second dielectric layers having strong dielectricconstant dispersion.
 2. The electro-optical modulator of claim 1,wherein the dielectric layers have an index of refraction less than orequal to 3.1 at a predetermined wavelength of operation of theMach-Zehnder interferometer.
 3. The electro-optical modulator of claim1, wherein the dielectric layers have relative dielectric constantgreater than or equal to 100 at a predetermined modulation frequency. 4.The electro-optical modulator of claim 1, wherein the Mach-Zehnderinterferometer further comprises: an input waveguide, a waveguidesplitter, two branch waveguides, a waveguide combiner, and an outputwaveguide, wherein each waveguide element comprises an intrinsic bulk ormulti quantum well structure (i-MQW) sandwiched between n-doped andp-doped semiconductor layers.
 5. The electro-optical modulator of claim4, wherein the first dielectric layer is disposed on the n-dopedsemiconductor layer along at least a portion of at least one of thebranch waveguides, and the second dielectric layer is disposed on thep-doped semiconductor layer along the portion of the at least one of thebranch waveguides.
 6. The electro-optical modulator of claim 5, furthercomprising: a first conductor overlaying the first dielectric layeralong the portion of the at least one of the branch waveguides, thefirst metal layer connected to a ground conductor of the coplanarwaveguide; and a second conductor overlaying the second dielectric layeralong the portion of the at least one of the branch waveguides, thesecond metal layer connected to a signal conductor of the coplanarwaveguide.
 7. The electro-optical modulator of claim 6, wherein athickness of the first dielectric layer and the second dielectric layeris sufficient to prevent an optical mode propagating in a branchwaveguide from overlapping the first metal layer and the second metallayer, respectively.
 8. The electro-optical modulator of claim 6,further comprising: a first ohmic contact connecting the first metallayer to the n-doped semiconductor layer along the portion of the atleast one of the branch waveguides; and a second ohmic contactconnecting the second metal layer to the p-doped semiconductor layeralong the portion of the at least one of the branch waveguides.
 9. Theelectro-optical modulator of claim 4, wherein the i-MQW comprisesInGaAlAs, and the n-doped and p-doped semiconductor layers comprisen-InP and p-InP, respectively.
 10. The electro-optical modulator ofclaim 1, wherein the first and second dielectric layers comprise one ofLiNbO₃, Ta₂O₅, and BaTiO₃.
 11. A method of fabricating anelectro-optical modulator comprising: forming a Mach-Zehnderinterferometer in a substrate removed semiconductor layer; and forming acoplanar waveguide overlaying the Mach-Zehnder interferometer, whereinsignals from the coplanar waveguide are capacitively coupled to theMach-Zehnder interferometer through first and second dielectric layershaving strong dielectric constant dispersion.
 12. The method offabricating an electro-optical modulator of claim 11, wherein thedielectric layers have an index of refraction less than or equal to 3.1at a predetermined wavelength of operation of the Mach-Zehnderinterferometer.
 13. The method of fabricating an electro-opticalmodulator of claim 11, wherein the dielectric layers have relativedielectric constant greater than or equal to 35 at a predeterminedmodulation frequency.
 14. The method of fabricating an electro-opticalmodulator of claim 11, wherein forming the Mach-Zehnder interferometerfurther comprises: forming an input waveguide, a waveguide splitter, twobranch waveguides, a waveguide combiner, and an output waveguide,wherein each waveguide element comprises an intrinsic bulk or multiquantum well structure (i-MQW) sandwiched between n-doped and p-dopedsemiconductor layers.
 15. The method of fabricating an electro-opticalmodulator of claim 14, further comprising: depositing the firstdielectric layer on the n-doped semiconductor layer along at least aportion of at least one of the branch waveguides, and depositing thesecond dielectric layer on the p-doped semiconductor layer along theportion of the at least one of the branch waveguides.
 16. The method offabricating an electro-optical modulator of claim 15, furthercomprising: depositing a first conductor overlaying the first dielectriclayer along the portion of the at least one of the branch waveguides,the first metal layer connected to a ground conductor of the coplanarwaveguide; and depositing a second conductor overlaying the seconddielectric layer along the portion of the at least one of the branchwaveguides, the second metal layer connected to a signal conductor ofthe coplanar waveguide.
 17. The method of fabricating an electro-opticalmodulator of claim 16, wherein a thickness of the first dielectric layerand the second dielectric layer is sufficient to prevent an optical modepropagating in a branch waveguide from overlapping the first metal layerand the second metal layer, respectively.
 18. The method of fabricatingan electro-optical modulator of claim 16, further comprising: forming afirst ohmic contact to connect the first metal layer to the n-dopedsemiconductor layer along the portion of the at least one of the branchwaveguides; and forming a second ohmic contact to connect the secondmetal layer to the p-doped semiconductor layer along the portion of theat least one of the branch waveguides.
 19. The method of fabricating anelectro-optical modulator of claim 14, wherein the i-MQW comprisesInGaAlAs, and the n-doped and p-doped semiconductor layers comprisen-InP and p-InP, respectively.
 20. The method of fabricating anelectro-optical modulator of claim 11, wherein the first and seconddielectric layers comprise one of LiNbO₃, Ta₂O₅, and BaTiO₃.